Amorphous Ammonia–Water Ice Deposited onto Silicate Grain: Effect on Growth of Mantles Ice on Interstellar and Interplanetary Dust

Abstract

The interaction of molecules on the mineral surface is interesting in understanding the development of icy mantles on interstellar and interplanetary dust. The ice grains can freeze and cover the silicate cores, growing an amorphous ice mantle. In the interstellar medium, olivine is a silicate that has been found in many places in dust. Previously we have simulated the interaction between amorphous water ice and forsterite surface. In this work we describe a more realistic situation, by adding ammonia molecules in a model of amorphous dirty ice onto forsterite surface. The NH3 is a part of the volatile components of cometary and interstellar ices. We propose models that describe a mixture of amorphous ice (ammonia–water) and forsterite (100) surfaces (dipolar and nondipolar). Our quantum mechanical calculations show that the ammonia has a similar affinity (30 kcal/mol) to the forsterite surface as that of water (31 kcal/mol). We calculated also the infrared frequencies to characterize the most reactive sites in the chemisorption processes. We observed important frequency shifts related to the position of the main vibrational modes of the NH3 moieties, which react chemically with the mineral surface.